Making a desk that open
I suspect that each part of your desk which articulates relative to any other part of the desk will need to be a separate object, and parented to some other appropriate part of the desk. But having written that , the exact method of associating the various parts of your desk so that they articulate as you expect in UE4 will be determined by the design of the UE4 game engine. I suggest that when you have completed all of the constituent parts of your desk that you ask about animating them on a forum devoted to UE4 topics.